Reference is made to the commonly assigned U.S. Patent Application, the respective disclosures of which being incorporated herein by reference:
U.S. patent application Ser. No. 09/580,185, filed on May 26, 2000, entitled xe2x80x9cFUSER LOADING SYSTEMxe2x80x9d.
This invention relates in general to a mechanism for controlling cam actuation, and more particularly to a cam control mechanism wherein when the cam stops in the wrong position a switch associated with the control mechanism is deactuated.
In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser device for typical electrographic reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser device rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member. In applying pressure to the fusing nip, the pressure must be held within a desired tolerance range in order to achieve adequate fusing without disrupting transport of the receiver member through the fuser device and without damaging the receiver member or the fuser device. Prior fuser devices have had difficulties in balancing these at-opposite requirements.
In order to accomplish proper pressure application in the fusing nip, a mechanism is fully described in the above-identified co-pending U.S. patent application Ser. No. 09/580,185, for applying pressure load force in a reproduction apparatus fuser device having at least one heated fuser member and a pressure member in nip relation to permanently fix a marking particle image to a receiver member. The pressure load force applying mechanism includes a load arm assembly rotatable about a fixed pivot axis to apply a pressure force to the pressure member, and a load cam selectively rotated about a drive shaft. A cam follower member is associated with the load cam, wherein upon rotation of the load arm assembly, a force of the load cam is applied via the cam follower member to the load arm assembly. A spring nest is formed as a part of the load arm assembly. The spring nest supports at least a heavy spring and a light spring. The cam follower member, upon movement under the influence of the load cam, compresses the nested light spring and the heavy spring at different travel positions of the cam follower for varying the pressure force on the pressure member.
For the described pressure applying mechanism, it was desired to minimize cost and maximize reliability. To do so, there were to be no critical adjustments and a minimum number of parts. It was, therefore, decided that only one switch should be used for producing control signals for the pressure control mechanism. Additionally, space constraints were placed on the size of the cam and motor. The pressure applying mechanism requires that the heavy and light springs be deflected an exact amount regardless of where the rotation of the load cam has stopped. The load cam has a large constant radius section to provide an exact deflection while allowing for switch and motor coast tolerances. In order to actuate the cam within the time allowed, the motor speed and gear set were chosen. With this gear set, the motor coasted when turned off, and a brake was needed to limit the motor coast. The cam follower needed to contain a low friction bearing so as to limit the drag on the motor and keep the motor small. Reliability of this mechanism has been less than generally acceptable. This is due to the fact that when a receiver member passes through the fuser rollers, the load arm will deflect slightly. Over long runs, the cam will rotate at an almost imperceptible rate, but eventually the follower will exit the constant radius section of the cam and fall off the high load.
The cause of the cam motion has been identified as being due to small tolerances in the parts that placed the follower off center of the cam pivot. The off center load causes a moment to be generated which attempts to rotate the cam. Generally, the moment was small enough to be resisted by the friction in the system. However, the holding friction in the ball bearing of the follower is small. As the pressure arm assembly pulsates with the passage of a receiver member through the fuser device, the fluctuating moment sometimes overcomes the friction in the mechanism and small movements ensue. Eventually, the cam will move far enough for the follower to exit the constant radius portion of the cam. Once the follower is on a rising section of the cam, the tangential force from the follower rapidly moves away from the cam pivot centerline, and the moment increases dramatically. This high torque causes the cam to rotate away from the desired position. The solution to this problem has been complicated because the moment applied to the cam was deemed to be inevitable with parts tolerances, and the follower needed to retain the ball bearing to keep motor torque low.
In view of the above, this invention is directed to A mechanism for applying pressure load force in a reproduction apparatus fuser device having at least one heated fuser member and a pressure member in nip relation to permanently fix a marking particle image to a receiver member. The pressure load force applying mechanism includes a load cam selectively rotated about a drive shaft, the cam having a wide constant radius section to have a wide tolerance in the stopping position. A cam follower member is associated with the load cam. A force of the load cam is applied via the cam follower member. A control mechanism is provided for the load pressure applying mechanism. The control mechanism includes a raised section at each end of the constant radius section of the load cam to act as stops for the follower.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.